As restorative materials for mending the defect of bone such as maxillary bone and biodegradable materials used as pins for osteosynthesis at a bone fracture there are disclosed biodegradable polymers such as polylactic acid, polyglycolic acid and polycaprolactam and copolymers thereof in JP-A-5-42202 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-5-309103.
The materials disclosed in the above cited patents are biodegradable, but their biodegradability can normally be hardly controlled. These polymers are also disadvantageous in that the enhancement of biodegradability requires the reduction of their molecular weight that deteriorates the mechanical properties thereof and hence causes a practical problem.
These biodegradable polymers are further disadvantageous in that their biodegradability is accelerated with time, causing the rise in the internal acidity thereof that tends to result eventually in the inflammation of the embedded site. These biodegradable polymers are further disadvantageous in that fibrous tissues or osseous tissues can hardly be induced into and regenerated at the site where they have been decomposed.
The complexing of a polylactic acid with hydroxyapatite or tricalcium phosphate in molten state is disclosed in JP-A-63-89166. A composite of an oligomer of lactic acid and/or glycolic acid with calcium phosphate is disclosed in JP-A-4-500013.
However, the complexing of a polylactic acid with hydroxyapatite or tricalcium phosphate in molten state is disadvantageous in that these materials can hardly be kneaded with each other, resulting in the production of a heterogeneous composite which can be hardly reproduced. Further, since a polylactic acid normally has a poor thermal stability, it is liable to drastic drop of molecular weight during melting, resulting in the production of a composite having a low elastic modulus and insufficient flexural strength and other physical properties. The application of such a composite is relatively restricted.
Referring to the composite of an oligomer of lactic acid and/or glycolic acid with calcium phosphate, the two components can be better kneaded with each other. However, the composite thus obtained has a molecular weight as low as from about 200 to 10,000. Therefore, the composite exhibits a low mechanical strength and an insufficient elastic modulus and thus can hardly be put into practical use.
JP-A-4-279520 discloses chemical-releasing preparations obtained by a process which comprises compression-molding a chemical and a polymer such as lactic acid and glycolic acid or copolymer thereof, or melting these materials and then molding the mixture, and chemical-releasing preparations obtained by a process which comprises granulating the chemical-releasing preparations, mixing the preparations with a calcium phosphate-based compound such as hydroxyapatite and .beta.-tricalcium phosphate, and then compression-molding the mixture.
JP-A-6-298639 discloses spherical chemical-releasing preparations obtained by molding a composite of a chemical with a polymer such as lactic acid, glycolic acid and lactone or copolymer thereof and a calcium phosphate-based compound such as hydroxyapatite and .beta.-tricalcium phosphate in a laminated structure having different amounts of chemical and chemical-releasing preparations obtained by expanding the chemical-releasing preparations with a blowing agent.
However, these chemical-releasing preparations are disadvantageous in that their biodegradation rate is accelerated with time, making it difficult to control the biodegradation thereof. These chemical-releasing preparations must be capable of gradually releasing a chemical as well as must have an affinity for tissue. Further, these chemical-releasing preparations must be flexible when applied to muscle, etc. Moreover, if surgically embedded in the living body, these chemical-releasing preparations are preferably not needed to be taken out therefrom.
The addition of calcium phosphate to a polylactic acid makes it possible to a gradually releasable chemical which exhibits a high affinity for tissue and doesn't need to be taken out. However, as the added amount of the calcium phosphate-based compound increases, the kneading of these components becomes more difficult and the resulting preparations show a lower flexibility. In the case where a known lactic acid-based copolymer with glycolic acid or the like is used, if the content of calcium phosphate is increased, the resulting material exhibits an insufficient strength.
Further, the polylactic acid, copolymer thereof or composite thereof with a calcium phosphate-based compound disclosed in the above cited patents normally has a great content of a residual lactide derived from polylactic acid. The residual lactide undergoes ring opening to become a chain dimer of lactic acid or lactic acid that can decompose the polylactic acid or copolymer thereof. Thus, these biodegradable materials decompose rapidly and exhibit a poor storage stability or thermal stability. Further, the content of residual lactide varies widely from lot to lot. Therefore, these biodegradable materials have varied biodegradability and thus can hardly be reproduced.